Gap fill of metal stack in replacement gate process

ABSTRACT

A method for fabricating a semiconductor device comprises forming a replacement gate structure on a semiconductor layer of a substrate. The replacement gate structure at least including a polysilicon layer. After forming the replacement gate structure, a gate spacer is formed on the replacement gate structure. Atoms are implanted in an upper portion of the polysilicon layer. The implanting expands the upper portion of the polysilicon layer and a corresponding upper portion of the gate spacer in at least a lateral direction beyond a lower portion of the polysilicon layer and a lower portion of the spacer, respectively. After the atoms have been implanted, the polysilicon layer is removed to form a gate cavity. A metal gate stack is formed within the gate cavity. The metal gate stack includes an upper portion having a width that is greater than a width of a lower portion of the metal gate stack.

BACKGROUND OF THE INVENTION

The present disclosure generally relates to the field of semiconductors,and more particularly relates to improving gap fill of the metal stackin replacement gate processes.

As gate sizes decrease for replacement metal gates (RMG) it becomes moredifficult for conventional RMG processes to completely fill the metalgate stack. As device scaling increases, conventional solutions willbecome even more limited.

SUMMARY OF THE INVENTION

In one embodiment, a method for fabricating a semiconductor device isdisclosed. The method comprises forming a replacement gate structure ona semiconductor layer of a substrate. The replacement gate structure atleast comprising a polysilicon layer. After forming the dummy gatestructure, a gate spacer is formed on the replacement gate structure.Atoms are implanted in an upper portion of the polysilicon layer. Theimplanting expands the upper portion of the polysilicon layer and acorresponding upper portion of the gate spacer in at least a lateraldirection beyond a lower portion of the polysilicon layer and a lowerportion of the gate spacer, respectively. After the atoms have beenimplanted, the polysilicon layer is removed to form a gate cavitysurrounded by the gate spacer. A metal gate stack is formed within thegate cavity and in contact with sidewalls of the gate spacer. The metalgate stack comprises a upper portion having a width that is greater thana width of a lower portion of the metal gate stack.

In another embodiment, a semiconductor device is disclosed. Thesemiconductor device comprises a semiconductor layer formed on asubstrate. Silicide areas are formed on source and drain regions. Areplacement gate is formed over the semiconductor layer. The replacementgate comprises a upper portion having a width that is greater than awidth of a lower portion of the replacement gate stack.

In yet another embodiment, an integrated circuit is disclosed. Theintegrated circuit comprises a semiconductor device. The semiconductordevice comprises a semiconductor layer formed on a substrate. Silicideareas are formed on source and drain regions. A replacement gate isformed over the semiconductor layer. The replacement gate comprises aupper portion having a width that is greater than a width of a lowerportion of the replacement gate stack.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures where like reference numerals refer toidentical or functionally similar elements throughout the separateviews, and which together with the detailed description below areincorporated in and form part of the specification, serve to furtherillustrate various embodiments and to explain various principles andadvantages all in accordance with the present invention, in which:

FIG. 1 is a cross-sectional view of an initial semiconductor structureaccording to one embodiment of the present disclosure;

FIG. 2 is a cross-sectional view of the semiconductor structure after anactive area has been defined according to one embodiment of the presentdisclosure;

FIG. 3 is a cross-sectional view of the semiconductor structure after areplacement gate structure has been formed according to one embodimentof the present disclosure;

FIG. 4 is a cross-sectional view of the semiconductor structure aftersilicide areas have been formed according to one embodiment of thepresent disclosure;

FIG. 5 is a cross-sectional view of the semiconductor structure afterdisposable material later have been formed thereon according to oneembodiment of the present disclosure;

FIG. 6 is a cross-sectional view of the semiconductor structure showingan implantation process that expands an upper portion of a polysiliconlayer in the replacement gate structure according to one embodiment ofthe present disclosure;

FIG. 7 is a cross-sectional view of the semiconductor structure afterthe implantation process of FIG. 6 has been performed according to oneembodiment of the present disclosure;

FIG. 8 is a cross-sectional view of the semiconductor structure after acontact etch-stop liner and a dielectric layer have been formedaccording to one embodiment of the present disclosure;

FIG. 9 is a cross-sectional view of the semiconductor structure afterthe dielectric layer and a portion of the expanded polysilicon layerhave been etched and polished according to one embodiment of the presentdisclosure;

FIG. 10 is a cross-sectional view of the semiconductor structure afterthe replacement gate structure has been removed according to oneembodiment of the present disclosure;

FIG. 11 is a cross-sectional view of the semiconductor structure after areplacement metal gate has been formed according to one embodiment ofthe present disclosure;

FIG. 12 is an operational flow diagram illustrating one process forforming silicide regions according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

It is to be understood that the present disclosure will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present disclosure.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

Referring now to the drawings in which like numerals represent the sameof similar elements, FIGS. 1-11 illustrate various processes forimproving gap fill of the metal stack in a replacement gate process. Itshould be noted that one or more embodiments of the present inventionare applicable to both bulk substrate devices and silicon-on-insulator(SOI) devices. FIG. 1 shows a partially fabricated semiconductor device100 comprising a handle substrate 102, a buried insulator layer (e.g.,buried oxide (BOX)) 104, and a semiconductor layer 106. The handlesubstrate 102 can be a semiconductor substrate comprising a singlecrystalline semiconductor material such as single crystalline silicon, apolycrystalline semiconductor material, an amorphous semiconductormaterial, or a stack thereof. The thickness of the handle substrate 102can be, for example, from 50 microns to 1,000 microns, although lesserand greater thicknesses can also be employed. A buried insulator layer104 includes a dielectric material such as silicon oxide, siliconnitride, silicon oxynitride, or any combination thereof.

The thickness of the buried insulator layer 104 can be, for example,from 50 nm to 500 nm, although lesser and greater thicknesses can alsobe employed. The thickness of the semiconductor layer 106 can be, forexample, from 3 nm to 60 nm, and typically from 5 nm to 10 nm, althoughlesser and greater thicknesses can also be employed. The semiconductorlayer 106 can comprise any semiconducting material, including but notlimited to Si (silicon), strained Si, SiC (silicon carbide), Ge(geranium), SiGe (silicon germanium), SiGeC (silicon-germanium-carbon),Si alloys, Ge alloys, GaAs (gallium arsenide), InAs (indium arsenide),InP (indium phosphide), any combination thereof, as well as other III/Vor II/VI compound semiconductors and alloys thereof. Also, nFET and pFETdevices formed from the structure of FIG. 1 can include a semiconductorlayer 106 with different materials.

An active area 208 for the FET is defined within the semiconductor layer106 through pad-film deposition, patterning (e.g., by photolithography),and reactive-ion etching (RIE), as shown in FIG. 2. For example, a padoxide having a thickness of 2 nm to 10 nm is formed in an oxidationfurnace, and a pad nitride is deposited over the pad oxide usinglow-pressure chemical vapor deposition (LPCVD) or rapid-thermal chemicalvapor deposition (RTCVD). Photolithography and a nitride-oxide-siliconRIE are then performed to define the active area 208.

Next, the active area 208 is isolated, such as through shallow trenchisolation (STI). In this embodiment, STI is obtained through depositionof an STI oxide, densification anneals, and chemical-mechanicalpolishing (CMP) that stops on the pad nitride. For example, shallowtrench isolation structures can be formed by trenches extending from thetop surface of the semiconductor layer 106 at least to the top surfaceof the buried insulator layer 104, filling the trenches with adielectric material, and removing excess dielectric material from abovethe top surface of the top semiconductor layer 106. The STI structures210, 212 are formed above the BOX layer 104 that is continuous aroundthe active area 208. The pad nitride, along with any STI oxide remainingon the pad nitride, and the pad oxide are then removed (e.g., throughwet etching using hot phosphoric acid and HF).

A replacement (or dummy) gate structure 314 is then formed on the activearea 206 of the FET, as shown in FIG. 3. In this embodiment, thereplacement gate 314 comprises multiple layers of oxide, polysilicon,amorphous silicon, nitride, or a combination thereof. For example, FIG.3 shows that the replacement gate comprises a gate dielectric 316 formedon the active area 208 on the semiconductor layer 106 by, for example,chemical vapor deposition (CVD) processes, thermal oxidation, or wetchemical oxidation. In embodiments, the dielectric layer 316 can be anyhigh-k dielectric layer such as, for example, hafnium aluminum oxide,zirconium oxide, silicate, or any combination thereof in a stackstructure. A polysilicon layer 318 or any other disposable material suchas amorphous silicon is then formed on and in contact with thedielectric layer 316 using a deposition process such as CVD. Thisreplacement gate stack acts as a place holder for the actual gate stackto be formed after a gate expansion process is performed. Also, in someembodiment, an interfacial layer (not shown) is formed on and in contactwith the semiconductor layer 106 prior to forming the dielectric layer316. In this embodiment, the dielectric layer 316 is formed on and incontact with the interfacial layer. A hard mask 320 is formed on and incontact with the polysilicon layer 318 using, for example a CVD process.The hard mask 320 can comprise oxide, nitride, silicon nitride, and/orthe like.

A gate spacer 322 comprising a dielectric material (such as siliconoxide, silicon nitride, silicon oxynitride, or a combination of these)is formed on the sidewalls of the replacement gate stack 314 comprisingthe dielectric layer 314, polysilicon layer 318, and the hard mask 320.In the illustrated embodiment, the dielectric material is formed andthen reactive-ion etching is used to remove the dielectric materialexcept from the sidewalls of the replacement gate 314. It should benoted that the replacement gate 314 and the gate spacer 322 can beformed prior to forming the STI structures. Once the replacement gate314 and spacer 322 have been formed source and drain regions 324, 326and source and drain extension regions 328, 330 are formed within thesemiconductor layer 106. In one embodiment, these semiconductor portions324, 326, 328, 330 are formed by introducing electrical dopants such asboron (B), gallium (Ga), indium (In), phosphorous (P), arsenic (As),and/or antimony (Sb) by ion implantation, plasma doping, and/or gasphase doping employing various masking structures as known in the art.In one embodiment, a thermal anneal can be performed to activate anddiffuse the implanted ions so as to form the source/drain regions 324,326 and 428 and the source/drain extensions 328, 330, such as through aspike rapid-thermal anneal (RTA).

Silicide areas 432, 434 are formed for contacts on the source/drainregions 324, 326 of the FET, as shown in FIG. 4. In this embodiment, ametal is deposited on top of the source/drain regions 324, 326. Ananneal is then performed to form silicide, and then the metal isselectively removed. For example, the metal can be nickel, cobalt,titanium, platinum, or an alloy or combination thereof. FIG. 5 showsthat after the silicide areas 432, 434 have been formed a disposablematerial layer 536 is formed over and in contact with the STI structures210, 212, any portion of the semiconductor layer 106 (if any) betweenthe silicide areas 432, 434 and the spacer 322, and the silicide areas432, 434. The disposable material layer 536, in one embodiment, alsocontacts the sidewall 538 of the spacer 322. The disposable materiallayer 536 comprises a thickness or height protects the silicide areasfrom subsequent etching of the hard mask 320 formed on the replacementgate 314 and a subsequent implantation of germanium into the replacementgate 314. For example, the disposable material layer 536 can comprise athickness of greater than 200 A and less than the height of the gate.The disposable material layer 536 comprises a self-planarizing materialsuch as flowable oxide (FOX) or a spin-on glass, or can comprise anon-self-planarizing material. In one embodiment, the disposablematerial layer 536 can be deposited by spin-on coating of aself-planarizing material. In another embodiment, the disposablematerial layer 536 can be formed by deposition of a disposable materialby chemical vapor deposition, planarization of the deposited disposablematerial, for example, by chemical mechanical planarization (CMP), andby recessing the top surface of the planarized disposable material, forexample, by a recess etch, which can be a wet etch or a dry etch.

Once the disposable material layer 536 has been formed, a controlledetching process can be performed to remove the hard mask 320 and aportion of the spacer 322. The etching of the spacer 322 can stop at orbelow a top surface of the polysilicon layer 318. In one embodiment, theetching process is a selective reactive ion etching (RIE) process thatis selective to the material of the hard mask 320 and the spacer 322,and does not remove portions of the polysilicon layer 318. In oneembodiment, germanium atoms are implanted in an upper/top portion 640 ofthe exposed polysilicon layer 318 of the replacement gate 314 by ionimplantation in a direction indicated by arrows 642, as shown in FIG. 6.The germanium implantation process uses germanium atoms at a high dose(>10¹⁵ Ge atoms/cm²) and at a low energy (i.e., an energy level wherethe germanium atoms only penetrate through a top portion 640 of thepolysilicon layer of the replacement gate 314 and not into alower/bottom portion 644). The directions 642 can be vertical or tiltedbetween, for example, 5 and 45 degrees from vertical. Other angles areapplicable as well. As a result of this germanium implantation process,the top portion 640 of the polysilicon layer 318 expands laterally, asshown in FIG. 7.

As a result of the lateral expansion of the top portion 640, a width 746of the top portion 640 is greater than a width 748 of the bottom portion644 of the polysilicon layer 318, as shown in FIG. 7. In one embodiment,the top portion 640 of the polysilicon layer 318 is expanded laterallyat least 10%. In other words, the width 746 is at least 110% of thewidth 748. In addition, an upper region 750 of the top portion 640 ofthe polysilicon layer 318 extends above and over a top surface 752 ofthe spacer 322. In some embodiments, the top portion 640 of thepolysilicon layer 318 comprises angles sidewalls, whereas the bottomportion 644 of the of the polysilicon layer comprises verticalsidewalls. The expanded areas improve gap fill during a subsequent RMGprocess. For example, the funnel shape of the empty gate allows the Workfunction metal (WFM) and bulk metal (such as Al and W) the fill gate gapeasier. Otherwise, if gate top opening is small and pinched-off themetal cannot fill into the gate forms voids.

The disposable material layer 536 is then selectively removed by, forexample, an etching process. A contact etch-stop liner 854 is formedover the structure, as shown in FIG. 8. For example, a contact etch-stopliner 854 is formed over and in contact with the STI structures 210,212, any exposed portions of the semiconductor layer 106 between the STIstructures 210, 212 and the silicide areas 432, 434, any exposedportions of the semiconductor layer 106 between the silicide areas 432,434 and the spacer 322, the silicide areas 432, 434, and a portion ofthe replacement gate structure 314 (the nitride spacer 322 and portion750 of the polysilicon layer 318 implanted with Ge atoms that extendsabove the top surface 752 of the spacer 322). In embodiments, thecontact etch-stop liner 854 comprises, for example, nitride, and can beformed using any conventional deposition process such as, for example,CVD.

A dielectric layer 856 (e.g., an oxide layer, nitride layer, low-kmaterial or any suitable combination of those materials) is formed overthe entire structure. This dielectric layer 856 is then etched down tothe level of the top surface of the gate spacer 322, as shown in FIG. 9.This process removes the portion 750 of the polysilicon layer 318 thatextends above the top surface 752 of the spacer 322, and thecorresponding portion of the contact etch-stop liner 854. Then, thereplacement gate 318, 316 is removed via selective etching or anothertechnique to form a gate cavity 1058 that exposes a portion 1060 of thehigh-k dielectric layer 316 or a portion of the semiconductor layer 106if the high-k dielectric have not been formed, as shown in FIG. 10. Thegate cavity 1058 comprises a top portion 1062 that has been laterallyexpanded as a result of the lateral expansion of the polysilicon layer318. A width 1064 of the top portion 1062 of the cavity 1058 is greaterthan a width 1066 of the bottom portion 1068 of the cavity 1058, both ofwhich correspond to the top and bottom portions 640, 642 of thepolysilicon layer 318 that has been removed. In one embodiment, the topportion 512 of the gate cavity layer is expanded laterally at least 10%.In other words, the width 1064 is at least 110% of the width 1066. Insome embodiments, the top portion 1062 of the gate cavity 1058 comprisesangled sidewalls, whereas the bottom portion 1068 of the gate cavity1058 comprises vertical sidewalls.

Once the polysilicon layer 318 has been removed, an RMG process isperformed. For example, a high-k dielectric material is blanketdeposited, for example by CVD (chemical vapor deposition), PECVD (plasmaenhanced chemical vapor deposition), or ALD (Atomic layer deposition).The excessive high-k gate dielectric above the dielectric layer 856 canbe removed, for example, by polishing such as chemically mechanicalpolishing (CMP) and/or etching to form a high-k gate dielectric layer1170 on and in contact with the dielectric layer 316 (if formed), thevertical sidewalls 1172 of the spacer 322, and the angled sidewalls 1174of the spacer 322.

Examples of high-k materials include but are not limited to metal oxidessuch as hafnium oxide, hafnium silicon oxide, hafnium siliconoxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide,zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide,titanium oxide, barium strontium titanium oxide, barium titanium oxide,strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandiumtantalum oxide, and lead zinc niobate. The high-k may further includedopants such as lanthanum, aluminum.

One or more conductive materials are then deposited on the high-k gatedielectric layer 1170 and etched/polished to form a metal gate 1176. Themetal gate 1176 fills the remaining portion of the gate cavity 1058. Inone embodiment, the conductive material comprises polycrystalline oramorphous silicon, germanium, silicon germanium, a metal (e.g.,tungsten, titanium, tantalum, ruthenium, zirconium, cobalt, copper,aluminum, lead, platinum, tin, silver, gold), a conducting metalliccompound material (e.g., tantalum nitride, titanium nitride, tungstensilicide, tungsten nitride, ruthenium oxide, cobalt silicide, nickelsilicide), carbon nanotube, conductive carbon, or any suitablecombination of these materials. The conductive material may furthercomprise dopants that are incorporated during or after deposition. Theconductive material may comprises multiple layers such as gate workfunction setting layer 1178 (work function metal) and gate conductivelayer. In some embodiments, the dielectric layer 1170 is not formed andthe gate work function setting layer is formed on and in contact withthe dielectric layer 316 (if formed), the vertical sidewalls 1172 of thespacer 322, and the angled sidewalls 1174 of the spacer 322.An optionalbarrier layer such as TiN can be inserted into the interface between theWFM 1178 and the high-k gate dielectric layer 1170. Contacts (not shown)can then be formed for the silicide areas 432 and 434. One or moreprocesses can be used to form the contacts.

It should be noted that although FIGS. 1-11 show only one semiconductordevice being fabricated, embodiments of the present disclosure areapplicable to fabricating multiple semiconductor devices (e.g., multiplenFETs or pFETS and/or nFETs and pFETS). It should be also noted thatembodiments of the present disclosure are not limited to the processesdiscussed above with respect to FIGS. 1-11. Embodiments of the presentdisclosure are applicable to any semiconductor device and fabricationprocess that implements a replacement metal gate.

FIG. 12 is an operational flow diagram illustrating one process forforming a semiconductor device with a replacement metal gate comprisingan expanded upper portion according to one embodiment of the presentinvention. In FIG. 12, the operational flow diagram begins at step 1202and flows directly to step 1204. It should be noted that each of thesteps shown in FIG. 12 has been discussed in greater detail above withrespect to FIGS. 1-11. A replacement gate structure, at step 1204, isformed on a semiconductor layer of a substrate. The replacement gatestructure comprises at least a polysilicon layer. After replacement gatestructure has been formed, a gate spacer is formed on the replacementgate structure, at step 1206. Atoms, at step 1208, are implanted in anupper portion of the polysilicon layer. The implanting expands the upperportion of the polysilicon layer and a corresponding upper portion ofthe gate spacer in at least a lateral direction beyond a lower portionof the polysilicon layer and a lower portion of the gate spacer,respectively. After the implanting, the polysilicon layer is removed toform a gate cavity surrounded by the gate spacer, at step 1210. A metalgate stack, at step 1212, is formed within the gate cavity and incontact with sidewalls of the gate spacer. The metal gate stackcomprises a upper portion having a width that is greater than a width ofa lower portion of the metal gate stack. Additional fabricationprocesses such as contact formation can then be performed. The controlflow exits at step 1214.

Although specific embodiments of the disclosure have been disclosed,those having ordinary skill in the art will understand that changes canbe made to the specific embodiments without departing from the spiritand scope of the disclosure. The scope of the disclosure is not to berestricted, therefore, to the specific embodiments, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentdisclosure.

It should be noted that some features of the present disclosure may beused in one embodiment thereof without use of other features of thepresent disclosure. As such, the foregoing description should beconsidered as merely illustrative of the principles, teachings,examples, and exemplary embodiments of the present disclosure, and not alimitation thereof.

Also that these embodiments are only examples of the many advantageoususes of the innovative teachings herein. In general, statements made inthe specification of the present application do not necessarily limitany of the various claimed disclosures. Moreover, some statements mayapply to some inventive features but not to others.

What is claimed is:
 1. A method for fabricating a semiconductorstructure, the method comprising: forming a gate spacer on a dummy gatestructure comprising a polysilicon layer; and implanting atoms in anupper portion of the polysilicon layer, the implanting expanding theupper portion of the polysilicon layer and a corresponding upper portionof the gate spacer in at least a lateral direction beyond a lowerportion of the polysilicon layer and a lower portion of the gate spacer,respectively.
 2. The method of claim 1, wherein the atoms are germaniumatoms and are implanted at a dose that is greater than 10¹⁵ Geatoms/cm².
 3. The method of claim 1, further comprising forming thedummy gate structure, wherein forming the dummy gate structurecomprises: forming a dielectric layer on a semiconductor layer; andforming the polysilicon layer on and in contact with the dielectriclayer.
 4. The method of claim 1, further comprising: forming a sourceregion and a source extension region within a semiconductor layer;forming a drain region and a drain extension region within thesemiconductor layer; and forming silicide areas in the source and drainregions.
 5. The method of claim 4, further comprising: forming aprotective liner over the silicide areas, the gate spacer, and the dummygate structure; and forming a dielectric layer over and in contact withthe protective liner.
 6. The method of claim 1, further comprising:after the implanting, removing the polysilicon layer to form a gatecavity surrounded by the gate spacer; and forming a metal gate stackwithin and conforming to the gate cavity and in contact with sidewallsof the gate spacer.
 7. The method of claim 6, wherein forming the metalgate stack comprises: depositing a dielectric material within the gatecavity to form a gate dielectric layer conforming to sidewalls of thegate spacer; and depositing one or more conductive materials on the gatedielectric layer to form a metal gate.
 8. The method of claim 6, furthercomprising: prior to depositing the one or more conductive materials,forming a work function metal layer conforming to the dielectric layer.9. The method of claim 6, further comprising: prior to depositing theone or more conductive materials, forming a work function metal layerconforming to sidewalls of the gate spacer.
 10. The method of claim 6,wherein the metal gate stack is formed with an upper portion having awidth that is greater than a width of a lower portion of the metal gatestack.
 11. A method for fabricating a semiconductor structure, themethod comprising: forming dummy gate structure comprising a polysiliconlayer; expanding an upper portion of the polysilicon layer and acorresponding upper portion of the gate spacer in at least a lateraldirection beyond a lower portion of the polysilicon layer and a lowerportion of the gate spacer, respectively.
 12. The method of claim 11,wherein the expanding comprises: implanting atoms in the upper portionof the polysilicon layer.
 13. The method of claim 12, wherein the atomsare germanium atoms and are implanted at a dose that is greater than10¹⁵ Ge atoms/cm².
 14. The method of claim 11, further comprisingforming the replacement gate structure, wherein forming the replacementgate structure comprises: forming a dielectric layer on a semiconductorlayer; and forming the polysilicon layer on and in contact with thedielectric layer.
 15. The method of claim 11, further comprising:forming a source region and a source extension region within asemiconductor layer; forming a drain region and a drain extension regionwithin the semiconductor layer; and forming silicide areas in the sourceand drain regions.
 16. The method of claim 15, further comprising:forming a protective liner over the silicide areas, the gate spacer, andthe replacement gate structure; and forming a dielectric layer over andin contact with the protective liner.
 17. The method of claim 11,further comprising: after the expanding, removing the polysilicon layerto form a gate cavity surrounded by the gate spacer; and forming a metalgate stack within and conforming to the gate cavity and in contact withsidewalls of the gate spacer.
 18. The method of claim 17, whereinforming the metal gate stack comprises: depositing a dielectric materialwithin the gate cavity to form a gate dielectric layer conforming tosidewalls of the gate spacer; and depositing one or more conductivematerials on the gate dielectric layer to form a metal gate.
 19. Themethod of claim 17, further comprising: prior to depositing the one ormore conductive materials, forming a work function metal layerconforming to the dielectric layer.
 20. The method of claim 17, furthercomprising: prior to depositing the one or more conductive materials,forming a work function metal layer conforming to sidewalls of the gatespacer.